This invention relates to a shift assisting device for a marine propulsion unit and more particularly to an improved arrangement for facilitating shifting without causing adverse engine operating characteristics.
In many forms of marine propulsion devices, such as outboard motors or inboard-outboard drives, there is provided a forward, neutral, reverse transmission for selectively operating the watercraft in forward, neutral or reverse conditions. Such transmissions normally employ a driving gear that is meshed with a pair of oppositely rotating driven gears that are selectively coupled to the driving shaft by means of dog clutch assemblies. The dog clutch assemblies are particularly useful in permitting the transmission of high driving loads and insuring against jumping out of gear. However, the shifting of the dog clutching elements presents some problems. For that reason, it has been the practice to provide some form of shift assist mechanism that will assist in shifting of the dog clutch into engagement by reducing the speed of the engine, if the engine speed is greater than a predetermined speed.
Normally, these shift assist mechanisms interrupt the ignition circuit of the engine to slow the engine speed during shifting operations and when the engine speed is above a predetermined value. Although such an arrangement has considerable utility, it also presents certain disadvantages under some circumstances. For example, this type of device may complicate the ignition circuit. In addition, even though the engine speed is slowed by interrupting the engine firing, there is the problem of unburned fuel that enters into the combustion chambers and then is exhausted through the exhaust system. Backfires can frequently occur with arrangements of this type due to the passage of large quantities of unburned fuel into the exhaust system, particularly when the engine speed is returned to normal.
It is, therefore, a principal object of this invention to provide an improved shift arrangement for a marine propulsion device.
It is a further object of this invention to provide an improved arrangement for assisting in shifting in a marine propulsion device without causing backfiring.
The aforenoted type of systems for shift assisting, as have been noted, require the use of ignition interruption. However, in certain types of engines such as diesel engines, there is not an ignition system per se. Therefore, the type of shift assist device previously employed cannot be utilized with diesel engine marine units.
It is, therefore, a further object of this invention to provide an improved shift assisting device that may be utilized with a wide variety of engine types including diesel engines.